1. Field of the Invention
This invention relates to a chip resistor which can be suitably used for detecting a small current. The present invention also relates to a current detecting circuit and method utilizing such a chip resistor. The present invention further relates to a method of adjusting the resistance of such a resistor.
2. Description of the Related Art
As is well known, chip resistors are used in various applications and have proven to enable high integration (high-density mounting). For the convenience of description, reference is now made to FIGS. 17 through 20 showing three different prior art chip resistors.
As shown in FIGS. 17 and 18, a typical prior art chip resistor comprises an insulating chip substrate a on which a resistor element b is formed by printing a resistor material paste. The resistor element b is connected endwise to a pair of electrode terminals c formed by printing a conductive paste. Further, the resistor element b is covered by a glass coating d for protection.
FIG. 19 shows another prior art chip resistor which differs from the resistor of FIGS. 17 and 18 only in that it comprises a shorter resistor element b. Such a chip resistor is advantageously usable as a current detector in a protection circuit for a DC/DC converter for example because it is capable of providing a low resistance of say 0.1.OMEGA..
FIG. 20 shows a further prior art chip resistor wherein a very narrow resistor element b is formed of a conductive paste integrally with electrode terminals c by simultaneous printing. Apparently, the resistor element b made of a conductive paste is suitable for providing a very low resistance.
According to either one of the prior art arrangements, the resistance of the chip resistor is determined for resistance adjustment by the so-called "four terminal method" as shown in FIG. 21. In FIG. 21, reference sign R1 represents the resistance of the resistor element b, whereas reference signs R2, R3 indicate the respective resistances of the two electrode terminals b.
As shown into FIG. 21, voltage detecting probes p are brought into contact with the respective electrode terminals c. In this condition, a current of a known value is allowed to flow across the electrode terminals c, and the voltage drop across the voltage detecting probes p is measured.
According to the method described above, since the two electrode terminals c are used for supplying the current and for detecting the voltage drop, it is impossible to determine the resistance of the resistor element b alone. Indeed, the measured voltage drop only represents the sum of the resistances R1, R2, R3. Further, since the resistances R2, R3 (which are very small) of the electrode terminals c are influenced not only by the conditions of thick film printing but also by solder deposits used for mounting the chip resistor, it is extremely difficult to equalize the resistance characteristics from one chip resistor to another.
The above-describe problem is particularly remarkable when the chip resistor is intended for providing a very low resistance because the small resistances R2, R3 of the electrode terminals c become more significant in determining the overall resistance of the chip resistor. Further, difficulty is also encountered when using the chip resistor as a detector for accurately measuring a small current.